Calliactis polypus (Forskål, 1775) is a sea anemone in the family Hormathiidae. It is a commensal species, usually associated with several species of hermit crab and can be found down to 25 metres in the Red Sea, Indian Ocean and tropical Pacific Ocean. (Ross, 1970, Fautin, 2008) Commensalism of C. polypus and Dardanus sp. has been explored. 

Anemones are opportunistic carnivores that catch prey as it swims or drifts into its tentacles or oral disc. C. polypus has similar anatomy and physiology to other Cnidarians, with the exception of the acontia, which few species possess. The acontia are threadlike defensive organs, composed of mostly nematocysts, discharged from the mouth or special pores during the retraction-deflation sequence, when irritated.(Farlex, 1913) The retraction-deflation sequence is an important defence tool for anemones as it keeps the soft parts of their body away from predators. The nematocysts specific to C. polypus were also investigated, as they are a defining character of the phylum. 

Autofluorescence is an important aspect of how cnidarians communicate with with reef coinhabitants. The specific patterns of autofluorescence function as a signalling mechanism to specific organisms capable of detecting the wavelengths emitted by cnidarians, much like how flowers use the UV spectrum to communicate with insects. (Gruber et al., 2008) Specimens were collected from Frenchman’s Beach, Stradbroke Island. Findings are included from studies into the acontia, autofluorescence, and the retraction-deflation sequence of C. polypus.

Calliactis polypus specimens attached to pumice stone in a rock pool at Frenchman's Beach, Point Lookout, North Stradbroke Island.

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